Place a rack in the lower third of your oven and preheat to 450°F. Drizzle a large rimmed baking sheet with 1½ tablespoons of the oil, brushing it so that it coats the pan.
Scrub and peel the potatoes if desired (I leave the peels on). Cut into ¼-inch-wide sticks. Place in a large bowl, cover with very hot water so that the potatoes are covered by 1 inch. Let sit 10 minutes. Drain the potatoes, then use a clean towel to dry them as completely as you can.
Rinse and wipe out the bowl you used to soak the potatoes, then return the potatoes to the bowl. Drizzle with the remaining 1½ tablespoons olive oil and sprinkle with Cajun seasoning, garlic powder, dill weed, and onion powder. Toss to coat, making sure the spices and oil are evenly distributed, then spread the potatoes into a single layer on the oiled baking sheet.
Roast in the lower third of the oven for 15 to 20 minutes, until turning golden underneath. Remove from the oven and, with a big spatula, gently loosen the fries from the pan, then flip them over in large sections. You don't need to fuss over flipping every single one, just make sure they're nicely rotated so they brown evenly. Spread back into a single layer, then continue roasting until they are golden and crisp as you like, about 5 to 10 additional minutes (we love crispy fries and bake them for 30 minutes).
While the fries bake, make the cheese sauce: In a wide saucepan, melt the butter over medium heat. Add the flour and whisk it constantly for 1 full minute. Slowly add the milk, stirring constantly as you go so the sauce is smooth. Bring to a gentle boil and stir constantly for 1 minute; the mixture should reduce somewhat.
Slowly stir in the cheese a little bit at a time (don't add too much at once or it will clump).
Transfer the fries to a serving plate (or eat them right off of the sheet pan). Drizzle with the cheese sauce and Ranch dressing (if desired). Sprinkle with chives. Enjoy immediately.
Notes
TO MAKE IN THE AIR FRYER: Toss the sliced potatoes with 2 tablespoons oil and the spices (you won't need the third tablespoon oil). Place half of them in your air fryer basket. Air fry for 10 to 12 minutes until crisp, shaking the basket several times throughout, until they are crisped to your liking. Repeat with the remaining fries (you can keep batches warm in a 300°F oven if you like). Top with the cheese sauce and chives.
TO STORE: This is not a recipe that stores well (and I definitely wouldn’t attempt to freeze it). But if you can’t eat all of this deliciousness in one go, transfer your leftovers to an airtight container and refrigerate for up to 3 days.
TO REHEAT: Warm leftovers in a 400ºF oven until the fries and cheese sauce are heated through. The microwave also works, but the fries will be soggy.